If the surface over which an air cushion vehicle travels is uneven, the vehicle may pitch or roll or undergo a combination of both motions. Such motions may even become extreme depending on surface unevenness, vehicle speed, etc.
The problem was recognized and attempts made to solve it in the prior art. Thus Pont et al U.S. Pat. No. 3,811,527, May 21, 1974, discloses an air cushion vehicle having means for creating a central or main air cushion including a surrounding flexible skirt and compartmental peripheral cushions including a flexible skirt surrounding each peripheral cushion which are all arranged about the periphery of the central cushion. The skirt surrounding the central cushion may be common to both it and the peripheral cushions. The supply of air to the several cushions is variable independently as to each cushion to control the attitude of the vehicle and/or "the angular speeds of pitch and roll . . . to improve the comfort of the passengers, the performance of the vehicle and the behavior of the skirts."
Jaumotte U.S. Pat. No. 3,587,772 Jun. 28, 1971, discloses an air cushion vehicle having two or more air cushions arranged in line longitudinally and/or transversely of the vehicle and spaced inwardly of the periphery thereof to provide a "first lifting stage". Each cushion is created by an endless channel-like inward directed nozzle supplied with an independent source of air under pressure, the lower ends of the nozzle being defined by flexible skirts. Depending from the periphery of the vehicle is a flexible skirt which depends lower than the skirts of the air cushions and surrounds all those cushions to receive air therefrom and provide a "second lifting stage".
The air cushion vehicles of both of the above referred to patents, however, have flexible skirts surrounding their several air cushions and also rather complicated constructions, especially that of Pont. The disadvantages of flexible skirts are well known.